Change fake profile behavior

If no average depth is known the current fake profile behavior is rather
unintuitive (we make up an average depth).

Instead we should assume that this is a PADI style dive log and give the
user a "rectangular" profile (actually, it's a trapecoid as we at least
try to enforce a sane ascent / descent speed). If the dive is somewhat
longer or deeper (10 min / 10 m) we even add a 3m safety stop at 5m.

Added a new dives/test0b that tries to capture the typical cases to test
this.

Fixes #398

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>